Service composition refers to a technology, where end user services to be provided to user terminals, e.g. to personal computers, laptop computers, or mobile phones of a telecommunications network, are dynamically built by combining constituent services. The selection of the constituent services as components of the composite service may be performed just in time at user request. The needed constituent services are described in terms of required generic properties. Any service that provides the needed properties can be used as part of the composite service. Rather than fixed binding of a particular service any suitable service within a pool of available services can be selected. The pool of available constituent services can change dynamically by adding new services or by removing them. A composite service can therefore consist of different constituent services at each invocation. Thus, the set of component services that are actually included into a composite service may not be static, but depending on runtime conditions. The constituent services do not need to be specifically designed for service composition. They can be integrated into a composite service, but they can also work as a single service.
From the end user point of view, a telecommunication network is substantially defined by the services it provides. The network user selects desired services from a portfolio of available services. For billing, a so-called central charging system is provided that is informed by the service providing nodes and applications about user activities. The charging system determines the amount to be charged for the service usage and deducts from the user's account (online charging) or it logs the activity in detail records for later billing (offline charging). The main information used by the charging system is the identification of the user and the details of the provided service. Various protocols exist to transfer this information from the service nodes to the charging system. Examples are for online charging protocols are DIAMETER or RADIUS. The roles in charging of the central charging system and service nodes are well standardized for example in the 32.xxx series of technical specification by 3GPP.
For billing of the end user, the charging system independently evaluates each service provide to the end user terminal. This approach of separate service processing is reasonable as long as the service price does not depend on the other services provided to the user or on the context of a service usage.
In a service environment that is based on service composition, it is usually not transparent to the end user that his activity starts a composite service that triggers the invocation of a number of implicit services. In other words, the end user expects the composite service being treated as one service item. However at the end, such end user will be charged according to all the services as if the were separate services. E.g. a post-paid subscription user would see the component services on his bill without being able connect them to the composite service he has ordered.